In electrophotographic image formation, an image formation method using a laser beam is used widely in order for acquiring a high-density image quality. In the electrophotographic image formation, the commonly used method of forming an electrophotographic latent image on a photoconductor drum is that the surface of the photoconductive drum is optically scanned in the axial direction of the photoconductive drum (main scanning) by a laser beam output from a polygon mirror at the same time the photoconductor drum is rotated (sub-scanning).
In the electrophotographic image formation, formation of a high-density image at high speed is demanded. If the image resolution is doubled in order to perform a high-density image formation, the time needed for the main scanning and the time needed for the sub-scanning must be doubled, so that the total time needed for outputting a reconstructed image would be 4 times as much as before. Therefore, in order to perform a high-density image formation, it is necessary to increase the image output speed as well.
As a conceivable method for increasing the image output speed, the use of a laser beam with higher output level, the use of multiple laser beams, and the use of a photoconductor with higher photosensitivity may be taken into consideration. Especially, in a high-speed image forming device, a multi-beam light source which outputs multiple laser beams is commonly used. The area where a latent image is formed on the photoconductor in a case where “n” laser beams are used simultaneously is increased n times as large as that in a case where a single laser beam is used, and the time needed for the image formation can be reduced to 1/n.
Japanese Laid-Open Patent Application No. 11-340570 discloses a photoelectric conversion device which includes a plurality of photoelectric conversion parts arranged on a substrate. Japanese Laid-Open Patent Application No. 11-354888 discloses a semiconductor light emitting device which includes a plurality of light emitting parts arranged on a substrate. Each of these devices has a one-dimensional arrangement in which end-face emitting semiconductor lasers are arranged in one direction. In these cases, if the number of laser beams increases, the power dissipation becomes large, and it is necessary to arrange a cooling system for cooling the lasers. Practically, the limitation may be the device using four beams or eight beams in view of the cost.
On the other hand, as the research has been conducted briskly in recent years, surface-emitting lasers can be easily integrated on a substrate in a two-dimensional formation. The power dissipation of a surface-emitting laser is smaller than that of an end-face emitting laser, and it is advantageous that a large number of surface-emitting lasers can be arranged in a two-dimensional formation.
Japanese Laid-Open Patent Application No. 2005-274755 discloses an image forming device including an exposure unit which causes a charged surface of an electrophotographic photoconductor to be exposed to laser light so that an electrostatic latent image is formed on the photoconductor. This exposure unit has a surface-emitting laser array and is of multi-beam type in which the photoconductor surface is scanned by three or more light beams to form an electrostatic latent image on the photoconductor.
Japanese Laid-Open Patent Application No. 2005-234510 discloses an optical scanning device which includes a plurality of light sources (main light source, sub light source) for emitting a plurality of light beams to irradiate a scanned surface of a photoconductor, and a light source driving unit for driving the plurality of light sources. The optical scanning device controls the distribution of the quantity of light in the sub-scanning direction on the scanned surface of the photoconductor by the light beams output from the light sources by using the light source driving unit. For this purpose, the quantity-of-light ratio between the main light source and the sub light source is varied without changing the total quantity of light from the two light sources (main light source, sub light source) and without causing the quantity of light from the sub light source to exceed the quantity of light from the main light source.
Japanese Laid-Open Patent Application No. 2001-272615 discloses an optical scanning device including an optical deflector which deflects a plurality of light beams emitted from a light source unit to perform the scanning of the scanned surface of the photoconductor by the deflected light beams. The sub-scanning of the scanned surface of the photoconductor is performed by moving the scanned surface relative to the deflector in the sub-scanning direction perpendicular to the main scanning direction. In the light source unit, the light emitting points are arranged in a two-dimensional formation and rotated on the same plane by a predetermined angle in the main scanning direction or the sub-scanning direction so that the respective scanning lines by the light beams are arranged on the scanned surface at equal intervals.
The surface-emitting laser array according to the related art has a problem that, if the spacing between two of the light emitting parts is shortened, the laser output performance may decline or the reliability may fall due to the influence of heat generated in other light emitting parts.